This invention relates to a selectively heat-treated article, such as a screw, threaded article, or the like and to a method and machine for making the same.
In the manufacture of thread-forming screws of hardenable metal, it is a common practice to case harden the screws so that the thread thereon will be effective to swage or cut a thread in the workpiece. Case hardening involves heat treating in a special atmosphere, usually one that is enriched in carbon. In accordance with case hardening techniques, the screw or other workpiece is heated through its entire body, and during the heating the carbon penetrates a predetermined distance into the body. Upon quenching in water or oil the carburized skin forms a hardened shell or case. A mild temper for stress relief may follow the quenching operation.
It is also known to harden and temper thread-forming devices (e.g. self-tapping screws, taps) by simply hardening in an atmosphere that contains sufficient carbon to prevent reduction of the carbon content of the surface of the thread. However, the process still involves a heating of the entire device to the requisite treatment temperature.
Conventional hardening processes of the foregoing type, and others of the prior art, do not always produce a thread-swaging screw of desired properties. The hardening may be satisfactory so far as producing a thread that is able to withstand thread-forming pressure in the workpiece; however, it often results in a screw that has undesirable structure and/or composition for certain purposes. In this regard it should be noted that in many thread-swaging fasteners, only a portion of the thread actually is used to form the thread while the remainder of the thread is for holding the screw and workpiece assembled. Thus, where the screw, when assembled with the workpiece, is subjected to dynamic stresses or even high static stresses, the surface of the screw may be subject to cracking due to the inherent brittleness. This may result in spreading of the crack through the body of the fastener and ultimate failure thereof.